pink/purple seems to be the color associated with culture Civ 5. (and Civ 4) Its the color your culture accumulation is listed in at the top, and the color of the "enact social policy" button.
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WillfulWizardSep 29 '10 at 0:39

3 Answers
3

Unlike previous versions of Civilization, in Civ V, territory around a city expands one tile at a time. The pink highlighted tiles are the ones that are next in line for expansion through culture. Consequently, they are also usually the cheapest tiles to purchase.

Notice that when there are multiple tiles outlined in pink, the city will expand to one of them chosen at random.
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OakSep 28 '10 at 20:52

3

+1 Prior to this answer I just assumed it was a 'recommended' tile purchase even though it usually wasn't the best tile to buy.
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MarkSep 28 '10 at 23:25

A City can work tiles that are up to 3 tiles away from the City. As well as this, a Civilization can gain Resources e.g. Iron, that are up to 5 tiles away from the City. Sometimes the city will highlight in pink, a tile which is outside of the City's "workable" 3 tile area (i.e. outside of the zone that can be worked by the City) It will do this to reach Resources. This can mean that purchasing tiles suddenly becomes more expensive than normal, (because the pink tiles are usually the cheapest, and in this case the pink tile is too far away to be purchased.)
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DanD manJan 14 '11 at 22:34